1. Field of the Invention
The present invention relates to a process for preparing photoconductive cadmium sulfide. More particularly, the present invention is concerned with an improved process comprising firing cadmium sulfide particles twice, thereby preparing photoconductive cadmium sulfide which is high in the photo-response speed and does not change in the photoconductive characteristic regardless of difference in the storage state therefor.
2. Description of the Prior Art
Photoconductive cadmium sulfide (CdS) is used typically as a photosensitive material for electrophotography. In the most general method of preparing it, hydrogen sulfide is caused to act on a water-soluble cadmium salt such as cadmium sulfate, cadmium chloride and the like to precipitate cadmium sulfide particles, which are then fired at a high temperature in order to dope the particles with an activator. More particularly, the cadmium sulfide particles are mixed with CuCl.sub.2, CuSO.sub.4 or the like as an activator and a halide such as CdCl.sub.2, ZnCl.sub.2 or the like as a flux, and fired to dope the particles with Cu, Cl and the like, thereby obtaining photoconductive cadmium sulfide. In an alternative method, a water-soluble cadmium salt is previously mixed with an acceptor and donor in the form of CuSO.sub.4, HCl and the like, and hydrogen sulfide gas is then introduced into the mixture to deposit cadmium sulfide particles, which already contain Cu and the like as the acceptor and Cl and the like as the donor. In this case, the cadmium sulfide particles can be doped sufficiently with the donor and acceptor impurities in the subsequent firing step even when the particles are fired in the absence of a flux.
However, in either method, the impurities remain in the vicinity of the surface of the precipitated CdS particles. The impurities cannot be removed by simple washing with water. In the former method, the unreacted starting material such as for example CdSO.sub.4 remains in the CdS particles. In the latter method, besides CdSO.sub.4, excessive Cl, Cu and the like which are not doped into the CdS particles in the firing step remain in the surface of the particles. These impurities remaining in the vicinity of the CdS surface cause occurrence of the surface defect of the CdS in the firing step. The surface defect becomes the trap level of the photo-carrier, and therefore, the photo-memory of the CdS is increased, that is, the photo-response speed of the CdS is deteriorated. When a photosensitive member using such a CdS is applied to a high speed copying machine, the electrostatic contrast between the light and dark portions becomes insufficient. In such a case that a photosensitive member is prepared by using the CdS particles, the resistance of the photoconductive layer changes and a difference is produced in the photodecay characteristic depending upon the storage conditions for the photosensitve member, that is, whether the photosensitive member is irradiated with light or not. If the electrophotographic property of the photoconductive layer considerably changes depending upon the difference in the storage conditions, it becomes impossible to form stable images. More particularly, the quality of the image which is formed by using a photosensitive member which has been left at the dark place becomes different from that of the image formed by using the photosensitive member which has been left at the light place. For example, the quality of the image obtained when the copying operation starts with the copying machine which is not operated for a considerably long period of time becomes different from that of the image which is obtained, successively after a good many of copies are made with the copying machine in a short time, by carrying out the copying operation with the same copying machine. Therefore, it becomes impossible to form images with stable image quality.
The copying speed becomes higher as the electrophotographic technique is improved. If the photodecay of the photosensitive member is slow, influence of the light irradiation applied in the first cycle of the image forming process inadvantageously remains when the second cycle of the image forming process is carried out. Such light irradiation includes, for example, imagewise exposure, whole surface exposure and pre-exposure conducted prior to the charging step in order to eliminate the electrostatic image formed in the last image forming process.
Further, some of the impurities depositing on the vicinity of the surface of the CdS particles remain there even after the firing of the particles. The photosensitive member produced by using such a CdS provides only electrostatic images having remarkably low electrostatic contrast between the light and dark portions when such photosensitive member is placed under the conditions of high temperature and high humidity, which is due to synergetic function of the hygroscopicity of the impurities and the electroconductivity increased by the moisture absorption. Therefore, formation of images becomes difficult.
The foregoing description is made with reference to CdS for use in an electrophotographic photosensitive member. Also when CdS is used for other applications, for example an element of an exposure meter and photovoltaic element, it is required to have little photo-memory and be excellent in humidity resistance and durability.